If a user equipment (UE for short) needs to perform uplink communication with a cell of a base station, the UE needs to obtain a timing advance (TA) of the cell. A TA from a specific UE to a specific cell changes along with changes of factors such as a location of the UE; therefore, each time after obtaining a latest TA from a base station, the UE starts or restarts a timing advance timer (TAT). When the TAT times out, the UE deems that a previously obtained TA is no longer valid; therefore, uplink resource configuration is released, and all uplink transmission except a random access request is terminated, so as to avoid unnecessary uplink interference. If TAs from the UE to multiple cells are the same, the multiple cells belong to a same TA group. Cells belonging to a same TA group share a same TAT. Corresponding to a TAT maintained by the UE, the base station also maintains a timer. Each time after sending a latest TA to the UE, the base station also correspondingly starts or restarts the timer maintained by the base station; therefore, the base station can infer, by using a state of the timer maintained by the base station, a state of the corresponding TAT maintained by the UE.
If the UE needs to send new uplink data to the base station, the UE sends a scheduling request (SR) to a specific cell of the base station to request an uplink resource. If the base station does not respond to the SR, the UE may repeatedly send the SR, but the maximum number of repetition times cannot be exceeded. If the number of times the UE sends the SR reaches the maximum number of repetition times, the UE deems that an SR failure occurs. If the UE detects an SR failure, the UE releases uplink resource configuration, and initiates a random access request to the base station.
A physical uplink control channel (PUCCH) resource is also required for the UE to send the SR. In most cases, the base station configures and sends to the UE in advance, a PUCCH resource for sending an SR. However, because the PUCCH resource is relatively scarce, depending on conditions such as an uplink service of the UE, the base station may also not allocate in advance, to the UE, the PUCCH resource for sending an SR.
After uplink data arrives in the UE but the UE detects that a TAT times out, or detects that an SR failure occurs, or detects that no PUCCH resource is available for sending an SR, the UE initiates a contention-based random access process to the base station. In this process, the UE obtains an updated TA and an uplink grant (UL Grant), where the uplink grant corresponds to an allocated uplink resource. The UE may send the uplink data or control information such as a buffer status report (BSR for short) by using the uplink resource. Then, the base station may send a reconfiguration message to the UE, where the message includes uplink resource configuration, and the uplink resource configuration specifically includes at least one type of the following configuration: scheduling request (SR) configuration, sounding (Sounding) reference signal configuration, and channel quality indicator (CQI) reporting configuration. The UE sends an SR, a Sounding, and a CQI to the base station according to the uplink resource configuration. In a new type of network deployment, a UE can simultaneously communicate with more than two base stations, where one base station may be referred to as a primary base station, another base station may be referred to as a secondary base station, and the primary base station and the secondary base station are connected.
If the UE detects that a TAT of the secondary base station times out, or an SR failure of the UE occurs in the secondary base station, or the UE has no PUCCH resource for sending an SR to the secondary base station, according to the prior art, the UE can initiate a contention-based random access process only to the secondary base station, and then the secondary base station may send a reconfiguration message to the UE. However, a secondary base station in the prior art may not provide a reconfiguration function, thereby resulting in a failed reconfiguration process.
In addition, the contention-based random access process used by the UE has a relatively long delay, and easily fails because of collision, thereby delaying sending of uplink data, and impeding user experience.